1. Field of the Invention
This invention generally relates to piezoelectric thin-film resonators and filters, and more particularly, to a piezoelectric thin-film resonator with a dome-shaped cavity and a filter.
2. Description of the Related Art
With the rapid spread of wireless devices as represented by mobile telephones, there is an increased demand for small-sized and lightweight resonators and filters that include the resonators used in combination. These days, attention is being focused on piezoelectric thin-film resonators and filters that include the piezoelectric thin-film resonators used in combination, whereas dielectric filters and surface acoustic wave (SAW) filters have mainly been used so far. This is because the piezoelectric thin-film resonators have an excellent characteristic at, in particular, high frequencies, can be reduced in size, and can be fabricated in a monolithic device.
Film Bulk Acoustic Resonator (FBAR) is known as one type of the afore-described piezoelectric thin-film resonators. FBAR has, as main component parts, a body of laminated structure (composite membrane) that includes: an upper electrode (film); a piezoelectric film; and a lower electrode (film). A via hole or cavity is defined in a portion, below the lower electrode, where the upper electrode and the lower electrode oppose each other. Such via hole or cavity is formed by wet or dry etching the backside of a silicon substrate used as a device substrate, or is formed by wet etching a sacrifice layer arranged on the surface of the silicon substrate.
When a high-frequency electric signal is applied between the upper electrode and the lower electrode, elastic waves are excited by the inverse piezoelectric effect or generated by a distortion caused by the piezoelectric effect, in the piezoelectric film. The elastic waves are converted into electric signals. The elastic waves are wholly reflected by the surface of the upper electrode in contact with air and that of the lower electrode in contact with air, resulting in a longitudinal mode thickness excitation having a main displacement in a thickness direction. In the above-described device structure, the resonance occurs at a frequency in which a total film thickness H of the thin-film structure has an integral multiplication (n times) of ½ wavelength of an elastic wave, the thin-film structure having the main component parts composed of the upper electrode film, the piezoelectric film, and the lower electrode film, which are formed above the cavity. A propagation velocity V of the elastic wave varies depending on the material, and a resonance frequency thereof is given by F=nV/2H. By using such resonance phenomenon, it is possible to control the resonance frequency with the film thickness as a parameter and to fabricate the resonator and the filter of a desired frequency characteristic.
Here, aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), titanium (Ti), or the like may be used for a metal material of the upper or lower electrodes. Alternatively, the afore-described substances used in combination may be employed as a laminating material. In addition, aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), lead titanate (PbTiO3), or the like may be used for the piezoelectric film. In particular, it is desirable to use aluminum nitride (AlN) or zinc oxide (ZnO) with a main axis orientation of (002) plane. Furthermore, silicon, glass, GaAs or the like may be used for a device substrate.
As described above, the piezoelectric thin-film resonator with the above-described configuration, a via hole or cavity needs to be formed immediately below the lower electrode (or the dielectric film). Hereinafter, via hole denotes an opening that penetrates through from the backside of the substrate to the front surface thereof, and cavity denotes an air space existent in the vicinity of the substrate surface or existent immediately below the lower electrode film (or the dielectric film). Conventional piezoelectric thin-film resonators are categorized into the via hole type and cavity type.
FIG. 1 is a cross-sectional view schematically showing the configuration of a conventional piezoelectric thin-film resonator (conventional example 1) described in Electron. Lett., 1981, Number 17, pp. 507-509. In this configuration, on a (100) silicon substrate 11 having a thermally oxidized film (SiO2) 12, there is provided a stacked structure that includes an Au—Cr film serving as a lower electrode 13, a ZnO film serving as a piezoelectric film 14, and an Al film serving as an upper electrode 15. A via hole 16 is formed below the stacked structure. The via hole 16 is formed by anisotropic etching from the backside of the (100) silicon substrate 11, by use of KOH water solution or EDP water solution, which is a compound liquid that includes ethylene diamine, pyrocatechol, and water.
The via hole type of the piezoelectric thin-film resonator shown in FIG. 1 has the following drawbacks. Firstly, the above-described anisotropic etching utilizes the characteristic in which the etch rate of (100) plane of the silicon substrate is higher, to some extent, than that of (111) plane. Therefore, anisotropic etching is an effective method as far as (100) plane is a cut surface of the silicon substrate. Secondly, it is inevitable that the via hole has side walls of a tilt angle of 54.7 degrees, which is an angle formed by intersecting (100) plane with (111) plane. This cannot prevent the device size from becoming larger, and the via hole formed by etching one region of the backside of the silicon substrate widely, decreasing the mechanical strength. Thirdly, when the filter is configured in such a manner that the above-described multiple piezoelectric thin-film resonators are adjacently arranged, the resonators cannot be downsized respectively and the filter cannot be made small to a size suitable for practical use. Fourthly, the via hole formed in the silicon substrate serves as an obstacle in fabricating other devices such as inductance or capacitance on a single substrate, increasing difficulty in integration. Fifthly, a special consideration is needed to avoid damage in weak devices in the dicing process of separating the silicon substrate into the respective chips or in the packaging process of mounting the chip onto a package.
Meanwhile, the cavity type of piezoelectric thin-film resonator has a stacked structure that includes the upper electrode, the piezoelectric film, and the lower electrode (and a dielectric film, if necessary) is provided on the sacrifice layer, and the cavity is formed by etching away the sacrifice layer.
FIG. 2 is a cross-sectional view schematically showing the configuration of the afore-mentioned cavity type of the piezoelectric thin-film resonator (conventional example 2), as disclosed in Japanese Patent Application Publication No. 60-189307. In the afore-mentioned structure, a lower electrode 23, a piezoelectric film 24, and an upper electrode 25 are provided to form a stacked structure, on a substrate 21 having a thermally oxidized film (SiO2) 22. A cavity 26 is provided below the stacked structure. The cavity 26 is formed in such a manner that an island-shaped sacrifice layer of ZnO is patterned in advance, the afore-described stacked structure is provided on such patterned sacrifice layer, and the sacrifice layer arranged below the stacked structure is removed by acid.
In general, in the piezoelectric thin-film resonators that utilize the longitudinal mode thickness excitation such as FBAR, it is a precondition that the piezoelectric film has an excellent orientation in order to obtain an excellent resonance characteristic. In most cases, the cavity depth needs several μm to several tens μm in consideration of oscillation displacement and the deflection in the membrane part. The surface is, however, rough after such thick sacrifice layer is formed, and the orientation of the lower electrode 23 and that of the piezoelectric film 24 are degraded by a great amount, the lower electrode 23 and the piezoelectric film 24 being grown on the sacrifice layer. The stacked body that includes the upper electrode 25, the piezoelectric film 24, and the lower electrode 23 is provided on a bridge-shaped underlying film that protrudes upward from the SiO2 film 22. This causes a problem that the strength is not sufficient with respect to the mechanical vibration and the reliability is not sufficient in practical use.
FIG. 3 is a cross-sectional view schematically showing the piezoelectric thin-film resonator (conventional example 3) disclosed in Japanese Patent Application Publication No. 2000-69594, as a method of addressing the problem in the orientation. The stacked structure is configured such that a lower electrode 33, a piezoelectric film 34, and an upper electrode 35 are formed on a silicon substrate 31 having a thermally oxidized film (SiO2) 32, and a cavity 36 is formed below the stacked structure. Such configured piezoelectric thin-film resonator is fabricated as follows.
Firstly, a dent portion is formed by etching in one region of the surface of the silicon substrate 31. Next, the thermally oxidized film (SiO2) 32 is provided on the surface of the silicon substrate 31 to prevent phosphor in phosphorus silica glass (PSG) used as a sacrifice layer from dispersing in the silicon substrate 31. After PSG of the sacrifice layer is deposited, polishing and cleaning are performed for mirror finishing of the surface. Subsequently, the lower electrode 33, the piezoelectric film 34, and the upper electrode 35 are sequentially stacked, and PSG is lastly removed. In the afore-described fabrication method of the piezoelectric thin-film resonator, however, the fabrication costs are high. In addition, the fabrication method includes a troublesome polishing process that requires the process of removing the slurry residue, and the productivity is inferior due to a number of fabrication processes.